Lens-based multiple beam antennae are known to offer a viable and lower cost alternative to phased array antennae for use in a range of applications, both military and non-military. In particular, multiple beam antennae with electronically switched beams and spherical dielectric lenses are known which are able to produce a wide field of coverage while avoiding some of the engineering issues that can arise with phased array antennae.
In US 2003/0006941, a multiple beam antenna comprises a hemispherical dielectric lens with multiple associated switchably selectable antenna feed elements, the lens being mounted adjacent to a reflector and being operable to provide directional coverage.
Multiple beam antennae may use spherical or partially spherical dielectric lenses, e.g. hemispherical lenses, in particular lenses known as “Luneburg” lenses having a continuously varying or step-graded index profile. In a known arrangement, a so-called “virtual source” antenna comprises a half (hemispherical) Luneburg lenses mounted adjacent to a conducting ground plane. When signals are injected into the lens at a certain angle by one of a number of switchable radiating elements disposed around a portion of the lens, radiation emerges from the lens, is reflected off the ground plane, and re-enters the lens at a different angle, so simulating the effect of a virtual source of radiation as if a full spherical Luneburg lens were being used.
Several methods of fabricating Luneburg lenses, capable of operating at microwave frequencies, have been developed. The most common method uses a hemispherical shell construction yielding an approximate stepped or graded index profile.
U.S. Pat. No. 5,781,163 describes an antenna arrangement based upon hemispherical dielectric lenses arranged as a collinear array of half Luneburg lenses mounted on a common ground plane, providing a low profile, low radar cross section, high-gain antenna. Each hemispherical lens is fed by a single radiating feed element mounted on a feed arm. Beam pointing is achieved by rotating the ground plane and moving all radiating feed elements simultaneously along their feed arms.
In one particular type of large array of full or half Luneburg lenses, it has been proposed to build a radiometer with exceptionally high gain. The antenna in that case was designed to operate at low microwave frequencies, typically less than around 5 GHz. Although low radar cross section is not an issue at these frequencies, half Luneburg lenses may be preferred because the ground plane offers a way of mechanically supporting the weight of the lenses. Each lens may be fed by a single radiating element or clusters of elements that are mounted on feed arms and are mechanically steered.
In known arrangements above, in order to provide at least hemispherical coverage, a certain amount of mechanical steering is required to the antenna.